In this project we are coupling genetic study of selected families and twin pairs with epidemiologic, immunologic, serologic and neurochemical studies of disorders due to multiple factors such as multiple sclerosis and parkinsonism. This approach should clarify the etiology of these diseases, indicate individuals or populations at high risk and suggest a mechanism for prevention and treatment. To date, over 20 presumptive "Multiple Sclerosis" families and over 140 twin pairs with this condition have been ascertained. Over 165 twin pairs with parkinsonism have been ascertained.